348 



MOLLUSCA 



SUB-KINGDOM VI 



FIG. 590. 



Vertical section of the shell of Unio. e, b, a, a', the outer prismatic 

 layer, showing successive increments of shell growth ; c, c', the inner 

 lamellar strata. Highly magnified (after Carpenter). 



structure. The external layer is usually thin, .flexible, and dark-coloured, 

 chiefly composed of a horny substance termed conchiolin. This layer is 

 known as the epidermis, or more properly the periostracum ; it is not easily 

 corroded, and hence serves as a protection to the underlying calcareous 

 layers. The outer calcareous layer is composed of prisms of calcite arranged 

 more or less perpendicular to the external surface ; the inner layer is made 

 up of thin, more or less parallel lamellae of porcellanous or pearly texture, 

 disposed at right angles to the general direction of the prismatic layer, 



and exhibiting the minera- 

 logical characters of ara- 

 gonite (Fig. 590). Besides 

 the lamellar or prismatic 

 structure, many forms 

 show under the micro- 

 scope minute, sometimes 

 branched tubulation. 



The variations in shell 



Substance are SOine what 

 pharapfpristif of fliflfprpnt 



groups. I he prisms vary 

 greatly in size, the larger occurring in Inoceramus and Pinna, the smaller 

 in the Anatinidae and Myacidae. The prismatic layer is wholly absent in 

 the Chamiclae and many other Teleodesmacea ; in the Pectinidae and Limidae 

 the prismatic layer is feebly developed and often recognisable only in young 

 shells. In the Rudistae the prisms run nearly parallel with the outer surface. 

 As aragonite is more soluble than calcite, it frequently happens in fossil 

 shells that the layers composed of the former mineral have entirely dis- 

 appeared, leaving only the calcitic layers. Pearls are merely loose portions 

 of the inner layer secreted by the mantle surface, usually around foreign 

 bodies which have reached the interior of the shell and set up irritation there. 



In the majority of Pelecypods the valves form a nearly complete defence ; 

 in borers, burrowers, and a few degenerate types, the valves cover less and 

 less surface in proportion to that which is bare ; in a few the mantle is re- 

 flected so as to envelop more or less of the outer surface of the valves ; and 

 finally, in Chlamydoconcha, the valves are permanently internal, separately 

 encysted, with the ligament isolated and encysted between them. No 

 example is known of a Pelecypod absolutely destitute of valves in the adult 

 state. 



The valves of the shell are in general substantially equal ; but sometimes 

 they are unequal, especially in sessile or sedentary forms ; and rarely they 

 are spirally twisted, as in Stavelia and Spirodomus. The hinge or articuhis 

 comprises the whole articulating apparatus, hinge plate, teeth, ligament, etc. ; 

 the primitive hinge, which is coextensive with the ligament, is distinguished 

 by Hyatt as the cardo. The cardinal axis, or right line forming the axis of 

 revolution of the hinge, is parallel with the antero-posterior axis of the 

 animal (as determined by a line drawn through the mouth and posterior 

 adductor) in the ordinary Teleodesmacea; but in the winged Prionodesmacea, 

 such as Ostrea, Perna, etc., the two axes are at a considerable angle with each 

 other. 



The dental armature is usually situated on the dorsal margin, which for 



